Methionine is an essential amino acid in the animal diet. Methionine has been produced synthetically for an extensive period of time by various multi-step chemical synthesis employing acrolein, methyl mercaptan and cyanide as starting materials. There are two product forms: D,L-methionine and its hydroxyanalog. D-methionine is converted into the required L-isomer in vivo, unlike all other amino acids. The market for feed-grade methionine has been reported to be improving due to increased demand in poultry and more recently swine feed supplementation. The ability of the leading methionine producers (Degussa AG, Adisseo, and Novus) to meet the market demand hinges on raw material supplies. The intermediates acrolein and methyl mercaptan must be converted into 3-methylthiopropionaldehyde (MMP) and further into methionine using hydrogen cyanide. All three producers have plans for expansion of their methionine production facilities and integration with raw material production as well (Chem. Marketing Reporter Apr. 7, 2003).
The biosynthetic pathways for methionine (a member of the aspartate family of amino acids) have been studied in a number of organisms and show similarities as well as differences. The first dedicated step, acylation of homoserine is catalyzed by homoserine acyltransferase, and is ubiquitous in all organisms despite differences in the transferred acyl group. The product of metA catalysis is either acetylhomoserine or succinylhomoserine. Acylhomoserine is then converted to homocysteine via a transsulfuration or a direct sulfhydrylation pathway. Both pathways have been reported to be present and functional in yeast, fungi, green plants and the bacterium Corynebacterium glutamicum. E. coli possesses only the transsulfuration pathway. The transsulfuration pathway goes through cystathionine as an intermediate and utilizes cysteine as a sulfur donor. The direct sulfhydrylation pathway involves the direct incorporation of sulfide to the acylhomoserine. The last step in the pathway involves the conversion of homocysteine to methionine catalyzed by an homocysteine methyltransferase, encoded by the metE or metH genes.
Other important amino acids, such as lysine, threonine, and tryptophan are produced via fermentation for use in animal feed. Therefore, these amino acids can be made using glucose and other renewable resources as starting materials. Unfortunately, the production of methionine via fermentation has not been as successful and the chemical synthesis of methionine is still used today. This is in part due to the lack of an efficient engineered biosynthetic pathway for methionine production, and a suitable production host.
The following disclosure provides an improved methionine biosynthetic pathway, as well as production host.